In an article in a recent edition of Mathematics in School (May 1989), Ann Kitchen considers the mathematics of lifting bridges, among which is the bascule bridge shown below, where the counterpoise weight travels down the arc of a circle. This was chosen as a “simplification”, though even so an explicit analytical solution cannot be found. In her example the potential energy of the system was chosen to be the same in the closed and open positions, and there is then a position of unstable equilibrium in between.